Connectomics [1]} aims to produce a blueprint of
the connections between all neurons. Such maps promise to help
illuminate how the brain learns and how memories are formed. Active
connectome research focuses not only on the human
brain [2]} but also on the brains of
insects [3]}, [4]}.
The fruit fly Drosophila melanogaster has shown excellent
associative learning ability to link certain odorant stimuli (i.e.,
complex mixtures of odor compounds) to rewards or
penalties [5]}; the fly tends to recognize a mixture
of stimuli as if they were a unitary object [6]}, [7]}. Studies of the fruit fly nervous system (as well as
related studies in insects in general) have revealed that olfactory
signals from the antenna terminate in a so-called antennal lobe (AL) of
the brain, which is a bundle of \(\sim \) 50 discrete subunits known as
glomeruli. These glomeruli act as feature extractors, turning each
complex odor mixture a 50-digit code. Projection neurons (PNs) connect
each of these glomeruli code components to multiple of the \(\sim \) 2000
so-called Kenyon Cells (KC), which are located in a remote area of the
insect brain known as the mushroom bodies (MB's). Thus, the projection
from the AL to the MB represents a massive representational expansion.
These KCs in the MB are then decoded by smaller neuropils that lead to
actuation of the fly behavior. From a deep neural network perspective,
this architecture has the form of a single-hidden-layer network with
an input layer (the AL), a hidden layer (the MB), and several parallel
output layers (the downstream neuropils that control fly motion).
However, research into the topology of the AL–MB connections has
revealed that the inputs to each KC are randomized, where even isogenic
flies appear to express different connections [8]}.
Furthermore, feedback inhibition within the MB regulates excitability
of the KCs to small regions, thereby enforcing a sparse representation
of each odor across the MBs that, when artificially disrupted, greatly
reduces the ability of flies to discriminate between
odors [9]}. Thus, flies achieve neuronal function
through the involvement of only a small fraction of their
latent-variable space in the MBs [10]}, where
each of these latent variables are effectively a randomized combination
of a few components of each feature [5]}.
